Laminated veneer lumber (LVL) is composed of layers or plies of wood veneer with the layers bonded together by an adhesive resin. The woodgrain in the veneers is in the same direction for all plies so that higher strengths than solid wood is obtainable.
The present methods for preparing LVL include drying veneer sheets to a low moisture content primarily so that regular plywood resins can be applied without causing problems in the press stage. If the moisture level of the veneer sheets is too high then steam is formed when the LVL product is compressed in a press causing blowouts. The resulting board does not meet the strength requirements.
With new adhesives now available, it is not always necessary to dry veneer sheets down below about a 5% moisture content. This reduces the energy required for the drying step.
Laminated veneer lumber produced from Douglas fir and other high density wood veneers has an edge stiffness value in excess of 2,000,000 psi. This MOE FIGURE is required for structural uses. Furthermore, it has been found that an LVL produced from a low density wood such as spruce, pine or subalpine fir cannot meet the 2,000,000 psi MOE FIGURE. In many areas where Douglas fir and other high density woods are not always available there is a need to use other wood sources to produce an LVL product which does meet this required MOE standard. It has been found that utilizing mixed species of low density and high density woods cannot meet the same standard as that of Douglas fir. Laminated veneer lumber made from low density woods is well documented in the prior art.
Throughout the specification the term "low density" includes woods that fall in the range of about 0.2 to 0.4 gms/cu cm on a green volume basis. Examples are white spruce, subalpine fir and lodgepole pine. The term "high density" includes woods that fall in the range of about 0.4 to 0.6 gms/cu cm on a green volume basis. Examples are Douglas fir, larch, birch and maple.
Examples of typical wood densities on a green volume basis are as follows:
______________________________________ White Spruce 0.328 gms/cu cm Lodgepole Pine 0.385 gms/cu cm B.C. Interior Douglas Fir 0.440 gms/cu cm B.C. Coastal Douglas Fir 0.449 gms/cu cm B.C. Larch 0.549 gms/cu cm ______________________________________